The laser has now been adopted as a powerful transmitter in optical communication systems. The laser operating at frequencies on the order of 10.sup.15 Hz, greatly expands the information transmission capabilities of communicating systems, because the rate at which information is transmitted is proportional to the bandwidth of the carrier. Carrier bandwidths at optical frequencies are several thousands of times larger than ordinary high frequency communications carrier bandwidths, and therefore promises significant improvement in communication system capabilities. The high degree of beam collimation in lasers provides additional advantages in reducing the size of transmitter and receiver antenna. In the development of receivers the problem of selectivity, selectively discriminating signal information in the presence of wide background noise has been encountered. For example, a signal transmitted at or about 6 .times. 10.sup.14 Hz during daylight will be accompanied by ambient scattered sunlight and, at night by man-made emissions such as illumination or reflected moonlight.
In order to filter background noise from transmitted signals it is necessary to selectively filter the information carrier frequency. There have been several attempts of providing the required high selectivity in the visible spectrum such as optical interference filters and Fabry-Perot etalons. These highly selective filters depend on the multiple reflection and interference of light waves between parallel plates. In the case of the Fabry-Perot etalon, considerable spectral narrowing of the transmitted light is achieved, but the transmitted intensity is enormously reduced. To the disadvantage of the optical communications, both of these instruments must be used close to normal incidence and are not useful as high-Q filters for large aperture systems.